1. Field of the Invention
The present invention relates generally to devices for protecting electrical circuits, and particularly to devices for protecting electrical circuits having test indicators.
2. Technical Background
An electrical distribution system typically includes a circuit breaker, branch circuit conductors connected to the circuit breaker, and wiring devices connected to the circuit conductors. Wiring devices include receptacles, switches, and other such devices. Each wiring device includes line contacts that couple the device to the source of electrical power, and load contacts that provide power to a load, such as an appliance or a switch. Protective devices are employed in the electrical distribution system to protect a circuit from hazards that occur due to electrical faults. There are several types of protective devices, one type is commonly known as a ground fault circuit interrupter (GFCI), and the other is known as an arc fault circuit interrupter (AFCI).
As the name suggests, a GFCI includes interrupting contacts that disconnect the line contacts from the load, when a ground fault is detected. For example, most GFCIs will sense a fault when the line (e.g., the “hot” electrical lead) is shorted to ground. In the absence of a GFCI, a life threatening electric shock hazard may be present.
An arc fault is a high power discharge of electricity between two or more conductors, such as the line conductor and the neutral conductor. An AFCI also includes interrupting contacts that are configured to disconnect the line contacts from the load when an arc fault is detected. There are two types of arc faults. One type is a parallel arc fault, and the other is known as a series arc fault. A parallel arc fault may be caused by damaged insulation, such as from an overdriven staple. The current through this type of fault is not limited by the impedance of the appliance (load), but rather by the available current of the source voltage. The available current is established by the impedance of the conductors and terminals between the source of line voltage and the position of the fault, thus effectively across the line. A series arc fault may be caused by a break in the line or neutral conductors of the electrical distribution system, or may be at a loose terminal at a wiring device within the system. The current through this type of fault is limited by the impedance of the load. Since the fault is in series with the load, this type of fault is referred to as a “series arc fault.” In the absence of an AFCI, the current generated by the arc fault may ignite combustible materials and result in fire.
As noted above, protective devices include a circuit interrupter, or interrupting contacts, that disconnect the load terminals from the line terminals when a fault is detected. Thus, the protective device is provided with a sensor for sensing the fault, and a detector for establishing if the sensed signal represents a true hazardous fault, as opposed to a false positive caused by electrical noise. The detector drives a switch that actuates the circuit interrupter. The circuit interrupter typically includes a relay or trip mechanism that is operated by a solenoid responsive to the switch. A power supply may be required to furnish power to the sensor, detector, switch or solenoid. The power supply may derive power from the electrical distribution system through either the line terminals or load terminals.
The protective device includes electronic and mechanical components that may fail. Component failure may occur for a variety of reasons. For example, failure may occur because of the normal aging of electronic components. Mechanical parts may become corroded, or experience mechanical wear. Devices may fail because of mechanical abuse, or because they are overloaded when installed. Electrical power surges, such as from lightning, also may result in failure. The sensor, the detector, the switch, trip mechanism, and/or power supply may fail. As a result, the circuit interrupter may not function to remove power from the load during a fault condition.
In one approach that has been considered, protective devices are often equipped with a test button to determine the operating condition of the device. The test button may be actuated periodically to determine the condition of the device. The test result may be indicated is a variety of ways. For example, the test result indicator may be a button that is configured to pop out of the device, the test indicator may be a visual display, such as a lamp or a mechanical flag, or the test indicator may be an audible sound generated by an annunciator. Unfortunately, the above approach does not provide the user with any indication that the device is at the end of its useful life. Thus, what is needed is a protective device that includes an end-of-life indicator. It would be beneficial to provide an end-of-life indicator that is enabled when the trip mechanism fails to cause the circuit interrupter to remove power from the load side within a predetermined time interval.